In a storage phosphor computed radiography (CR) system, a photostimulable storage medium, such as a sheet or plate of storage phosphor, also known as a stimulable phosphor, is exposed to x-ray radiation that has been directed through an object or organ, such as a body part of a patient, to record a latent radiographic image in the storage phosphor. The latent radiographic image stored from the x-ray exam is then read out by stimulating the storage phosphor with stimulating radiation of a read wavelength, such as red or infra-red radiation, produced by a laser or other intense light source. Upon stimulation at the read wavelength, the storage phosphor emits stimulated radiation of an emission wavelength, such as blue radiation. In a CR system, the storage phosphor is typically scanned by a laser beam which provides the read wavelength and the stimulated radiographic image at the emission wavelength is detected by a photodetector to produce an electronic radiographic image. The image can then be digitized and stored, transmitted, or output on a display or recorded onto a radiographic film.
After it is read, the storage phosphor sheet, herein termed the CR plate or storage medium, is then erased so that it can be reused. Erasure of the stimulable phosphor material is typically performed using a high-intensity lamp that emits visible light having suitable spectral characteristics. Erasure uses a different range of wavelengths than those used for recording or reading the radiographic image. For example, high-brightness fluorescent light is often used for erasure. The newly erased CR plate is then stored back in its cassette, ready for re-use.
One problem with the CR plate relates to the sensitivity of the stimulable phosphors used on the CR plate. Even in a protected environment, the stimulable phosphors themselves, following erasure, are still sensitive to environmental radiation including cosmic rays, X-rays emitted from nearby X-ray sources, and trace radiation sources, including those on the CR plate itself and from its surroundings. Understandably, it would be a formidable task to formulate the stimulable phosphors so that they are less sensitive to such environmental radiation without compromising imaging performance at the same time. The problem of sensitivity to environmental radiation has other repercussions, including added noise content for diagnostic images. This, in turn, tends to increase the relative dose levels needed for diagnostic imaging in order to maintain acceptable signal-to-noise (SN) levels.
This sensitivity to environmental radiation limits the useful storage time or “shelf life” of the CR cassette following erasure. If the time interval since its last erasure is too long, imaging may not be satisfactory. Recognizing this problem, some CR imaging technologists or radiographers attempt to recycle and re-use CR cassettes in a sequential order according to relative erasure time. Using a cassette management sequence of this type, the CR cassette having the longest time since its last erasure is used first, with others sequenced in order of their relative time since last erasure. While manual record-keeping and cassette recycling practices have merit, however, they are prone to error.